Definition of Positron emission tomography

Positron emission tomography: PET. A highly specialized imaging technique that uses short-lived radioactive substances to produce three-dimensional colored images of those substances functioning within the body. These images are called PET scans and the technique is termed PET scanning.

PET scanning provides information about the body's chemistry not available through other procedures. Unlike CT (computerized tomography) or MRI (magnetic resonance imaging), techniques that look at anatomy or body form, PET studies metabolic activity or body function. PET has been used primarily in cardiology, neurology, and oncology.

The PET scan has been used to assess the benefit of coronary artery bypass surgery, identify causes of childhood seizures and adult dementia, and detect and grade tumors. It is very sensitive in picking up active tumor tissue but does not measure the size of it.

In PET the patient receives a short half-lived radiopharmaceutical (produced by a cyclotron or a generator). Because the radioisotope used in a PET scan is short-lived, the amount of radiation exposure the patient receives is about the same as two chest X-rays. The radiopharmaceuticals discharge positrons from wherever they are used in the body. As the positrons encounter electrons within the body, a reaction producing gamma rays occurs.

The patient lies on a table that slides into the middle of the scanner. Within the scanner are rings of detectors containing special crystals that produce light when struck by a gamma ray. The scanner's electronics record these detected gamma rays and map an image of the area where the radiopharmaceutical is located. Since the radiopharmaceutical contains a chemical commonly used by the body, PET enables the physician to see the location of the metabolic process. For example, glucose (or sugar, which the body uses to produces energy) combined with a radioisotope will show where glucose is being used in the brain, the heart muscle, or a growing tumor.